Optical measurement systems which use a laser triangulation sensor to obtain dimensional information about objects are known. For example, such systems may be arranged to determine the distance of objects from the sensor or the profile of objects within the field of view of the sensor.
Referring to FIG. 1, a conventional laser triangulation measurement device 1 comprises a light source 2 e.g. laser which is arranged to project a planar beam 4 (e.g. sheet) of light. The planar beam 4 is incident as a line 8 on an object or objects 5, 6 which lie in the field of view 7 of the device 1. Light reflected from the incident line 8 is collected by an imaging device 3, which may be a camera (e.g. having a charge coupled device (CCD) or an active pixel sensor (CMOS) device).
The images captured by the imaging device 3 are processed to determine a data representation of the physical geometry of the objects 5, 6. The processing may involved reference to a calibrated look up table or the like. Such processing is known.
FIG. 2 depicts two dimensions that may be determined using the data representation. The separation (gap G) or planar misalignment (mismatch or flush F) between adjacent surfaces may be determined, e.g. by performing suitable mathematical operations (e.g. line/radius fitting).
An example of a conventional optical triangulation sensor is the GapGun, manufactured by Third Dimension Software Limited.
It is known to use an optical triangulation sensor to measure the size of the spacing, i.e. gap, between the end of a turbine blade and the inner surface of the annular outer casing in which the turbine blade is rotatably mounted. Conventionally this is done with a wedge-shaped standoff that is jammed in between the blade and the surface of the casing to provide a reference surface for the sensor measurements. These measurements have to be performed for various blades around different positions on the casing. This limits the speed at which the measurements can be taken.
Other techniques for obtaining the similar information are also known. For example, U.S. Pat. No. 4,326,804 discloses a system for optically measuring the clearance between a rotating blade member and a relative stationary part. In this system, a probe that is fixed relative to the stationary part emits light towards the rotating blade. The light scattered back from the blade is measured in order to calculate the clearance.
U.S. Pat. No. 7,889,119 discloses a radial gap measurement technique for turbines in which a radar sensor is mounted in the wall of a turbine housing to transmit and receive a microwave signal. The radial gap can be calculated from an evaluation of a Doppler effect exhibited in the measured reflected microwave signals.